Non-aqueous pigment ink

ABSTRACT

A non-aqueous pigment ink comprising a pigment, a pigment dispersant and a non-aqueous solvent, wherein the pigment dispersant is an acrylic polymer comprising, as constituent monomers, (M1) an alkyl (meth)acrylate having an alkyl group of 12 or more carbon atoms, (M2) a (meth)acrylic acid derivative having an amino group, and (M3) (meth)acrylic acid and/or a (meth)acrylic acid derivative having a carboxyl group, in which a combination of all the constituent monomers comprises from 8 to 15 mol % of the monomer (M2) and from 0.6 to 8 mol % of the monomer (M3), and the molar ratio (M2)/(M3) between the monomer (M2) and the monomer (M3) is within a range from 1.3 to 15.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2007-325672 filed on Dec. 18,2007; the entire contents of which are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a non-aqueous pigment ink, and moreparticularly to a non-aqueous pigment ink that is suited to use withinan inkjet recording system.

BACKGROUND ART

An inkjet recording system is a printing system in which printing isconducted by spraying a liquid ink with a high degree of fluidity fromvery fine nozzles, and adhering that ink to a recording medium such as asheet of paper. These systems enable the printing of high-resolution,high-quality images at high speed and with minimal noise, using acomparatively inexpensive printing apparatus, and are rapidly becomingwidespread.

The coloring materials for the inks used in these inkjet recordingsystems can be broadly classified into materials that use pigments andmaterials that use dyes. Of these, there is a growing tendency for theuse of inks that use pigments as the coloring materials, as such inksexhibit the excellent levels of light resistance, weather resistance andwater resistance that are required for high image quality printing.

In terms of the solvent, inks can be broadly classified into aqueousinks and non-aqueous inks. In an aqueous ink, because an aqueous solventand water act as the ink medium, dispersing a pigment finely within thismedium and then maintaining the stability of that dispersion isextremely difficult.

As a result, aqueous pigment inks have been proposed in which thepigment is encapsulated, thereby enabling dispersion within the aqueousmedium (see Japanese Patent Laid-Open No. H09-151342 and Japanese PatentLaid-Open No. H11-140343). However, because the inks are water-based,the problem of poor water resistance is undeniable.

In contrast, non-aqueous inks that do not use water as the ink solvent,including solvent-based inks that use a volatile solvent as the mainconstituent and oil-based inks that use a non-volatile solvent as themain constituent, are now attracting considerable attention. Non-aqueousinks exhibit good drying properties to aqueous inks, and also offerexcellent printability.

These non-aqueous inks typically comprise a non-aqueous solvent, apigment, and a pigment dispersant and the like. Because the commonlyused pigment dispersants are polymers, interaction or cross-linkingbetween molecules tends to occur readily within the solvent. Theseinteractions or cross-linking between molecules of the pigmentdispersant prevent aggregation of the pigment and enhance thedispersibility, and are therefore necessary to a certain extent.However, if such interactions or cross-linking occur excessively, thenif the ink is left sitting in an open system for a long period, normaldischarge of the ink from the nozzles of the inkjet discharge headbecomes impossible (due to factors such as an increase in the number ofnon-discharging nozzles or a deterioration in the flight of the inkdroplets), and it is now known that these factors can cause whitebanding within the image or a deterioration in the image precision.

Moreover, particularly in those cases where the printer is left standingin a high-humidity high-temperature environment, it has been found thateven if the nozzle portions are subjected to a cleaning operation thatincludes pushing out and wiping off the ink, some nozzles are notrestored (do not return to their normal discharge state).

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide anon-aqueous pigment ink that is stable upon storage for a long period inan open system, is also stable upon storage in an open system within ahigh-humidity high-temperature environment, and exhibits excellentdischarge stability when used in an inkjet recording system.

A first aspect of the present invention provides a non-aqueous pigmentink comprising a pigment, a pigment dispersant and a non-aqueoussolvent, wherein the pigment dispersant is an acrylic polymercomprising, as constituent monomers, (M1) an alkyl(meth)acrylate havingan alkyl group of 12 or more carbon atoms, (M2) a (meth)acrylic acidderivative having an amino group, and (M3) (meth)acrylic acid and/or a(meth)acrylic acid derivative having a carboxyl group, the combinationof all the constituent monomers comprises from 8 to 15 mol % of themonomer (M2) and from 0.6 to 8 mol % of the monomer (M3), and the molarratio (M2)/(M3) between the monomer (M2) and the monomer (M3) is withina range from 1.3 to 15.

A second aspect of the present invention provides a printed item printedusing the non-aqueous pigment ink according to the aspect of the presentinvention described above.

A third aspect of the present invention provides a pigment dispersantfor a non-aqueous pigment ink composed of an acrylic polymer comprising,as constituent monomers, (M1) an alkyl (meth)acrylate having an alkylgroup of 12 or more carbon atoms, (M2) a (meth)acrylic acid derivativehaving an amino group, and (M3) (meth)acrylic acid and/or a(meth)acrylic acid derivative having a carboxyl group, the combinationof all the constituent monomers comprises from 8 to 15 mol % of themonomer (M2) and from 0.6 to 8 mol % of the monomer (M3), and the molarratio (M2)/(M3) between the monomer (M2) and the monomer (M3) is withina range from 1.3 to 15.

DESCRIPTION OF THE EMBODIMENTS

A non-aqueous pigment ink according to the present invention (hereafteralso referred to as simply “the ink”) uses, as a pigment dispersant, anacrylic polymer comprising a plurality of specific monomers (M1) to (M3)as constituent monomers. In other words, this acrylic polymer compriseslong-chain alkyl groups of 12 or more carbon atoms, amino groups andcarboxyl groups, and the quantities of the amino groups and carboxylgroups are controlled. As a result, a non-aqueous pigment ink can berealized in which both the degree of interaction and the cross-linkingdensity between molecules of the acrylic polymer that acts as thepigment dispersant can be controlled within suitable ranges, the storagestability of the ink is favorable, even in an open system and evenwithin a high-humidity high-temperature environment, and the dischargestability of the ink when used within an inkjet recording system isexcellent.

The ink according to the present invention comprises, as essentialcomponents, a pigment, a pigment dispersant, and a non-aqueous solvent.

Examples of pigments that can be used include organic pigments such asazo-based pigments, phthalocyanine-based pigments, dye-based pigments,condensed polycyclic pigments, nitro-based pigments and nitroso-basedpigments (such as brilliant carmine 6B, lake red C, Watchung red, disazoyellow, Hansa yellow, phthalocyanine blue, phthalocyanine green, alkaliblue, and aniline black); inorganic pigments, including metals such ascobalt, iron, chrome, copper, zinc, lead, titanium, vanadium, manganeseand nickel, as well as metal oxides and sulfides, and yellow ocher,ultramarine and iron blue pigments; and carbon blacks such as furnacecarbon black, lamp black, acetylene black and channel black. Thesepigments may be used either alone, or in combinations of two or moredifferent pigments.

From the viewpoint of ensuring favorable dispersibility and storagestability, the average particle size of the pigment is preferably notgreater than 300 nm, is even more preferably not greater than 150 nm,and is most preferably 100 nm or smaller. In this description, theaverage particle size of the pigment refers to the value measured usinga dynamic light-scattering particle size distribution measurementapparatus LB-500 manufactured by Horiba, Ltd.

The quantity of the pigment within the ink is typically within a rangefrom 0.01 to 20% by weight, and from the viewpoints of print density andink viscosity, is preferably within a range from 1 to 15% by weight, andeven more preferably from 5 to 10% by weight.

The pigment dispersant (hereafter also referred to as simply “thedispersant”) is an acrylic polymer comprising, as monomers, (M1) analkyl(meth)acrylate having an alkyl group of 12 or more carbon atoms,(M2) a (meth)acrylic acid derivative having an amino group, and (M3)(meth)acrylic acid and/or a (meth)acrylic acid derivative having acarboxyl group. Here, the meaning of the term “(meth)acrylate” includesboth acrylate and methacrylate, and the meaning of the term“(meth)acrylic acid derivative” includes both acrylic acid derivativeand methacrylic acid derivative.

Examples of the alkyl group of 12 or more carbon atoms contained withinthe monomer (M1) include a dodecyl group, tridecyl group, tetradecylgroup, pentadecyl group, hexadecyl group, heptadecyl group, octadecylgroup, nonadecyl group, eicosanyl group, heneicosanyl group or docosanylgroup. This alkyl chain at the ester portion may be either a straightchain or a branched chain.

Specific examples of the monomer (M1) include straight-chainalkyl(meth)acrylates such as dodecyl (meth)acrylate,docosyl(meth)acrylate, cetyl(meth)acrylate and stearyl(meth)acrylate,and branched-chain (meth)acrylates such as isostearyl(meth)acrylate.These monomers may be used either alone, or in combinations of two ormore different monomers. Of the various monomers, from the viewpoint ofachieving both a reduction in the ink freezing point and a reduction inthe viscosity of the pigment dispersion (the dispersion comprising thepigment, the pigment dispersant and a diluting solvent), the use ofdodecyl(meth)acrylate (lauryl(meth)acrylate) and/ordocosyl(meth)acrylate (behenyl(meth)acrylate) is preferred, and using acombination of these two monomers is even more desirable.

Including the monomer (M1) having an alkyl chain length of 12 or morecarbon atoms means that the ink can be used to provide a printed item ofhigh print density. From the viewpoints of ensuring favorable storagestability for the ink and maintaining favorable print density whenprinting to plain paper, the proportion of the monomer (M1) within thecombination of all the constituent monomers is preferably within a rangefrom 77 to 91.4 mol %, and even more preferably from 80 to 90 mol %.

Alkyl(meth)acrylates having an alkyl chain length of less than 12 carbonatoms may also be used in limited quantities, and examples of preferredcompounds include branched-chain(meth)acrylates such as2-ethylhexyl(meth)acrylate, isooctyl(meth)acrylate andtert-octyl(meth)acrylate. These compounds may be used either alone, orin combinations of two or more different compounds.

Comparatively short-chain alkyl(meth)acrylates such asmethyl(meth)acrylate, ethyl(meth)acrylate, butyl(meth)acrylate andoctyl(meth)acrylate may also used, but the resulting polymer tends to beprone to precipitation if these monomers undergo polymerization within anon-aqueous solvent, and these monomers also exhibit a strong odor andstrong skin irritancy, and therefore in those cases where thesecompounds are required, the quantity used should be restricted to thesmallest quantity possible.

In terms of the (meth)acrylic acid derivative having an amino group(M2), the use of a (meth)acrylic acid having a tertiary amino group ispreferred. The amino group is preferable as it is a functional groupthat exhibits strong adsorption of the pigment, and tertiary aminogroups are preferred as they exist stably within solvents. From theviewpoint of facilitating the approach to the pigment, this tertiaryamino group substituent is preferably a group that is not too stericallybulky.

For this reason, specific examples of compounds that can be usedfavorably as the monomer (M2) include dimethylaminoethyl(meth)acrylate,diethylaminoethyl(meth)acrylate, dimethylaminopropyl methacrylamide anddipropylaminoethyl(meth)acrylamide, each of which may be used alone orin combinations containing a plurality of compounds.

From the viewpoints of avoiding increases in the ink viscosity over timeand ensuring favorable dispersion stability for the pigment (favorablestorage stability for the ink), the proportion of the monomer (M2)within the combination of all the constituent monomers is preferablywithin a range from 8 to 15 mol %, and even more preferably from 8.5 to13 mol %.

The monomer (M3) is either (meth)acrylic acid or a (meth)acrylic acidderivative having a carboxyl group. Preferred examples of the latter,listed as readily available commercial products, include NK-Ester CB-1(β-methacryloyloxyethyl hydrogen phthalate), NK-Ester SA(β-methacryloyloxyethyl hydrogen succinate), NK-Ester ACB-100(β-acryloyloxyethyl hydrogen phthalate), NK-Ester ACB-200(β-acryloyloxypropyl hydrogen phthalate), and NK-Ester A-SA(β-acryloyloxyethyl hydrogen succinate), all of which are manufacturedby Shin-Nakamura Chemical Co., Ltd. These compounds may be used eitheralone, or in combinations containing a plurality of different compounds.

From the viewpoints of ensuring the long-term storage stability of theink and enabling favorable restoration of the head discharge propertiesfollowing standing within a high-humidity high-temperature environment,the proportion of the monomer (M3) within the combination of all theconstituent monomers is preferably within a range from 0.6 to 8 mol %,and even more preferably from 0.7 to 7 mol %. Although only supposition,it is thought that intermolecular cross-linking is generated viahydrogen bonding between the amino groups and the carboxyl groups (inother words, the amino groups perform both pigment adsorption andcross-linking functions). Accordingly, it is thought that by controllingthe quantities of these two. functional groups, the cross-linkingdensity between molecules of the pigment dispersant can be lowered, andas a result, any increase in the viscosity of the ink or precipitationof solid matter in the vicinity of the nozzles can be avoided, dischargefaults can be reduced, and the head discharge properties can befavorably restored.

Moreover, although only supposition, it is thought that molecules of thepigment dispersant undergo cross-linking and exist in an arrangementthat encapsulates the pigment adsorbed to the dispersant, and that as aresult, desorption of the pigment from the dispersant is less likely tooccur, thus enhancing the dispersibility of the pigment. At the sametime, it is thought that because the dispersion efficiency of thedispersant is improved, the quantity used of the dispersant can be keptat a suitable level, meaning adverse effects caused by the existence ofan excess of the dispersant can be avoided.

In addition, it is surmised that because the pigment dispersantcomprises amino groups and carboxyl groups, the dispersant itself is acompound of high polarity, and because this compound encapsulates thepigment, the solvent release properties for the pigment relative to thenon-aqueous solvent are enhanced, meaning printed items of high printdensity can be obtained, and show-through of the ink to the underside ofthe printed item can be prevented.

The molar ratio (M2)/(M3) between the monomer (M2) and the monomer (M3)is preferably within a range from 1.3 to 15, and is even more preferablyfrom 1.5 to 14. As described above, the amino group and the carboxylgroup are functional groups that undergo a mutual interaction, and byusing from 1.3 to 15 times as many amino groups as carboxyl groups, acombination of favorable restoration of the head discharge propertiesfollowing standing in a high-humidity high-temperature environment andfavorable long-term storage stability can be achieved.

Besides the above monomers (M1) to (M3), the acrylic polymer thatfunctions as the pigment dispersant may also include limited quantitiesof other monomers capable of copolymerization with these monomers.Examples of these other monomers include glycidyl (meth)acrylate,styrene, a-methylstyrene, maleate esters, fumarate esters,acrylonitrile, methacrylonitrile, vinyl acetate, α-olefins,N-methylpyrrolidone and styrene macromonomers. These monomers may beused either alone, or in combinations of two or more different monomers.

From the viewpoint of ensuring favorable ink discharge properties, themolecular weight (weight average molecular weight) of the acrylicpolymer is preferably within a range from approximately 15,000 to35,000, and is even more preferably from approximately 20,000 to 30,000.

The monomers described above can be polymerized using a conventionalradical copolymerization method. The reaction is preferably conducted aseither a solution polymerization or a dispersion polymerization.

In order to ensure that the molecular weight of the acrylic polymerfollowing polymerization falls within the preferred range describedabove, the use of a chain transfer agent during polymerization iseffective. Examples of compounds that can be used as this chain transferagent include thiols such as n-butyl mercaptan, lauryl mercaptan,stearyl mercaptan and cyclohexyl mercaptan.

Examples of polymerization initiators that may be used includeconventional heat polymerization initiators, including azo compoundssuch as AIBN (azobisisobutyronitrile), and peroxides such as t-butylperoxybenzoate and t-butylperoxy-2-ethylhexanoate (Perbutyl O,manufactured by NOF Corporation).

Petroleum-based solvents (such as aroma-free (AF) solvents) and the likecan be used as the polymerization solvent used in the solutionpolymerization. This polymerization solvent is preferably one or moresolvents selected from amongst those solvents that can be used, as is,for the non-aqueous solvent for the product ink.

In those cases where the acrylic monomer comprises a glycidyl group, thepolymerization temperature is preferably not too high in order toprevent ring-opening of the glycidyl group during the polymerization,and conducting the polymerization at a temperature of 65° C. or lower ispreferred. For this reason, the use of2,2′-azobis(2,4-dimethylvaleronitrile) (V-65, manufactured by Wako PureChemical Industries, Ltd.), which functions as an oil-soluble, lowtemperature, azo-based polymerization initiator, is particularlysuitable. Photopolymerization initiators that generate radicals uponirradiation with an active energy beam can also be used.

During the polymerization reaction, other typically employedpolymerization inhibitors, polymerization accelerators and dispersantsand the like may also be added to the reaction system.

From the viewpoint of pigment dispersibility, the blend quantity of thepigment dispersant within the ink, reported as a weight ratio relativeto 1 part of the pigment, is preferably within a range fromapproximately 0.05 to 1.0 parts, and is even more preferably from 0.1 to0.7 parts.

Relative to the total weight of the ink, the quantity of the pigmentdispersant is preferably within a range from approximately 0.5 to 10% byweight, and is even more preferably from 1 to 5% by weight.

The non-aqueous solvent refers to a non-polar organic solvent or polarorganic solvent for which the 50% distillation point is at least 150° C.The “50% distillation point” is measured in accordance with JIS K0066“Test Methods for Distillation of Chemical Products” and refers to thetemperature at which 50% by weight of the solvent is evaporated.

For example, examples of preferred non-polar organic solvents includealiphatic hydrocarbon solvents, alicyclic hydrocarbon solvents andaromatic hydrocarbon solvents. Specific examples of the aliphatichydrocarbon solvents and alicyclic hydrocarbon solvents include TecleanN-16, Teclean N-20, Teclean N-22, Nisseki Naphtesol L, Nisseki NaphtesolM, Nisseki Naphtesol H, No. 0 Solvent L, No. 0 Solvent M, No. 0 SolventH, Nisseki Isosol 300, Nisseki Isosol 400, AF-4, AF-5, AF-6 and AF-7,all manufactured by Nippon Oil Corporation; and Isopar G, Isopar H,Isopar L, Isopar M, Exxsol D40, Exxsol D80, Exxsol D100, Exxsol D130 andExxsol D140, all manufactured by Exxon Mobil Corporation. Specificexamples of the aromatic hydrocarbon solvents include Nisseki Cleansol G(alkylbenzene) manufactured by Nippon Oil Corporation, and Solvesso 200manufactured by Exxon Mobil Corporation.

Examples of preferred polar organic solvents include ester-basedsolvents, alcohol-based solvents, higher fatty acid-based solvents,ether-based solvents, and mixed solvents thereof. Specific examplesinclude ester-based solvents such as methyl laurate, isopropyl laurate,isopropyl myristate, isopropyl palmitate, isostearyl palmitate, methyloleate, ethyl oleate, isopropyl oleate, butyl oleate, methyl linoleate,isobutyl linoleate, ethyl linoleate, isopropyl isostearate, methylsoybean oil, isobutyl soybean oil, methyl tallate, isobutyl tallate,diisopropyl adipate, diisopropyl sebacate, diethyl sebacate, propyleneglycol monocaprate, trimethylolpropane tri-2-ethylhexanoate and glyceryltri-2-ethylhexanoate; alcohol-based solvents such as isomyristylalcohol, isopalmityl alcohol, isostearyl alcohol and oleyl alcohol;higher fatty acid-based solvents such as isononanoic acid, isomyristicacid, hexadecanoic acid, isopalmitic acid, oleic acid and isostearicacid; and ether-based solvents such as diethylene glycol monobutylether, ethylene glycol monobutyl ether, propylene glycol monobutylether, and propylene glycol dibutyl ether.

These non-aqueous solvents may be used either alone, or in mixtures oftwo or more different solvents.

In addition to the components described above, the ink may also include,as required, any of the various additives typically used within thefield, provided the inclusion of these additives does not impair theobject of the present invention.

Specific examples of these additives include anionic surfactants,cationic surfactants, amphoteric surfactants, nonionic surfactants, orpolymer-based, silicone-based or fluorine-based surfactants, which maybe added to the ink as antifoaming agents or surface tension reducingagents or the like.

By adding an antioxidant, oxidation of the ink components can beprevented, enabling the storage stability of the ink to be improved.Examples of antioxidants that may be used include conventionalantioxidants such as dibutylhydroxytoluene, propyl gallate, tocopherol,butylhydroxyanisole or nordihydroguaiaretic acid, and these may be usedeither alone, or in mixtures of two or more different antioxidants.

By adding a preservative, decomposition of the ink can be prevented,enabling the storage stability of the ink to be improved.

Moreover, by adding a polyoxyethylene alkylamine (an ethylene oxideadduct of an aliphatic amine: C_(n)H_(2n+1)N[(EO)_(x)H][(EO)_(y)H]) tothe ink, the discharge stability can be further improved, and a higherlevel of print density can be achieved when printing to plain paper. Inthe above general formula, EO represents an oxyethylene group, and x andy each represent, independently, an integer of 0 or greater, providedthat both are not zero.

Examples of the alkylamine within the above formula include laurylamine,stearylamine, and oleylamine. Of these, from the viewpoints of ensuringeven more favorable levels of discharge stability and low-temperaturestorage stability, laurylamine is preferred.

In terms of achieving superior discharge stability, high print densityon plain paper, and superior storage stability under low-temperatureconditions, the number of mols of added ethylene oxide (the combinedtotal of x and y in the above general formula) is preferably within arange from 2 to 8, and is even more preferably from 3 to 7.

In those cases where a polyoxyethylene alkylamine is added, from theviewpoints of achieving superior discharge stability and high printdensity on plain paper, the blend quantity of the polyoxyethylenealkylamine within the ink is preferably within a range from 1.0 to 5.0%by weight.

The ink can be produced by first preparing a pigment dispersioncomprising the pigment, the pigment dispersant and the non-aqueoussolvent, and subsequently adding additional non-aqueous solvent and anyother optional components. The non-aqueous solvent (or diluting solvent)used during preparation of the pigment dispersion is preferably the sameas the non-aqueous solvent incorporated within the ink, and in thosecases where the dispersant is synthesized by solution polymerization inthe manner described above, is preferably the same as the polymerizationsolvent.

The pigment dispersion comprising the pigment, the pigment dispersantand the non-aqueous solvent is preferably obtained by mixing the threecomponents together, and dispersing the pigment using an appropriatedispersion device such as a ball mill or beads mill.

The ideal range for the ink viscosity varies depending on factors suchas the diameter of the discharge head nozzles and the dischargeenvironment, but typically for use within inkjet recording devices, at23° C., is preferably within a range from 5 to 30 mPa·s, more preferablyfrom 5 to 15 mPa·s, and approximately 10 mPa·s in one preferableembodiment. Here, the viscosity is measured at 23° C. by raising theshear stress from 0 Pa at a rate of 0.1 Pa/s, and refers to the measuredvalue at 10 Pa.

A printed item according to the present invention is an item printedusing the ink of the present invention described above. By using an inkaccording to the present invention, problems such as non-dischargingnozzles and deterioration in the flight of the ink droplets can beprevented, and a sharp image with no white banding can be obtained, evenwhen printing is conducted using an inkjet recording system. Moreover,by using a pigment dispersant according to the present invention, thepigment is less likely to penetrate into the interior of the printingpaper, and accumulates more readily on the paper surface, meaning animage with a high print density can be obtained.

Although there are no particular restrictions on the printing method,conducting the printing using an inkjet recording apparatus ispreferred. The inkjet printer may employ any of various printingsystems, including piezo systems, electrostatic systems and thermalsystems. In those cases where an inkjet recording apparatus is used, theink according to the present invention is discharged from the inkjethead based on a digital signal, and the discharged ink droplets areadhered to a recording medium.

The ink according to the present invention can be used favorably evenunder low-temperature conditions, and exhibits excellent dischargestability when used in an inkjet recording system.

EXAMPLES

A more detailed description of the present invention is presented belowbased on a series of examples, although the present invention is in noway limited by these examples.

<Synthesis of Pigment Dispersant>

In a 500 ml four-necked flask were mixed lauryl methacrylate(manufactured by NOF Corporation), behenyl methacrylate (manufactured byNOF Corporation), dimethylaminoethyl methacrylate (manufactured by WakoPure Chemical Industries, Ltd.) and methacrylic acid (manufactured byWako Pure Chemical Industries, Ltd.), using the quantities (g) shown inTable 1. Subsequently 1.5 g of V-65 (manufactured by Wako Pure ChemicalIndustries, Ltd.) as a polymerization initiator, 1.2 g of stearylmercaptan (manufactured by Wako Pure Chemical Industries, Ltd.) as achain transfer agent and 230.9 g of AF7 (AF solvent No. 7, anaphthene-based solvent, manufactured by Nippon Oil Corporation) wereadded, and the resulting mixture was then reacted under reflux for 5hours under temperature conditions of 61° C.±3° C., thereby yielding asolution (solid fraction: 30%) of a pigment dispersant A. Followingreaction, a very small quantity (0.002 g) of methoquinone(p-methoxyphenol) was added as a polymerization inhibitor.

The polymer obtained by heating 10 g of the thus obtained solution ofthe pigment dispersant A on a hotplate at 120° C. for 5 hours weighed2.8 g (yield: 93%), and the weight average molecular weight (determinedusing a GPC method and referenced against polystyrene standards) of thepolymer was 19,500.

Using the constituent monomers shown in Table 1, the same method asabove was used to prepare solutions of pigment dispersants B to I (allof which were AF7 solutions having a solid fraction of 30% by weight).The diethylaminoethyl methacrylate and dimethylaminopropylmethacrylamide were the products manufactured by Wako Pure ChemicalIndustries, Ltd.

The polymer reaction yields were all within a range from 85 to 94%.

TABLE 1 Pigment Dispersant Formulations Constituent Molecular monomerblend weight (g) weight A B C D E F G H I Lauryl methacrylate 254 69.4667.95 67.72 70.78 71.27 67.20 67.57 69.25 69.84 Behenyl methacrylate 39522.54 22.05 21.98 22.97 23.13 21.81 21.93 22.47 22.66 Dimethylaminoethyl157 6.00 8.00 6.00 5.40 8.00 9.50 8.00 5.00 methacrylateDiethylaminoethyl 185 7.60 methacrylate Methacrylic acid  86 2.00 2.002.70 0.25 0.20 3.00 1.00 0.28 2.50 Combined weight of monomers (g) 100100 100 100 100 100 100 100 100 Proportion of C12 or higher alkyl 84.3281.33 81.64 89.12 90.23 78.84 81.68 85.87 84.51 (meth)acrylate (M1)within all monomers (mol %) Proportion of amino group-containing 9.7512.82 10.41 10.11 9.15 12.56 15.37 13.28 8.10 monomer (M2) within allmonomers (mol %) Proportion of carboxyl group-containing 5.93 5.85 7.950.77 0.62 8.60 2.95 0.85 7.39 monomer (M3) within all monomers (mol %)(M2)/(M3) molar ratio 1.64 2.19 1.31 13.15 14.79 1.46 5.20 15.65 1.10Polymer molecular weight 19500 24000 25500 20000 20000 27500 20500 2100025000

Example 1

In a 250 ml polypropylene container were mixed 45.0 g of the abovesolution of the pigment dispersant A, 27.0 g of a black pigment S170 (acarbon black, manufactured by Degussa AG, primary particle size: 17 nm),31.5 g of isooctyl palmitate (manufactured by Nikko Chemicals Co., Ltd.)and 31.5 g of AF7 (as described above), 450 g of zirconia beads(diameter: 0.5 mm) were then added to the mixture, and followingdispersion for 60 minutes using a rocking mill (manufactured by SeiwaTechnical Lab Co., Ltd.), the zirconia beads were removed by filtration,yielding a pigment dispersion.

To 135 g of the thus obtained pigment dispersion were added, asnon-aqueous solvents, 92.6 g of AF7 (as described above), 10.0 g of AF4(AF solvent No. 4, a naphthene-based solvent, manufactured by Nippon OilCorporation) and 62.4 g of isooctyl palmitate (as described above), andthe flask contents were then filtered through 3.0 μm and 0.8 μm membranefilters to remove any contaminants or coarse particles, thereby yieldingan ink of Example 1. The pigment content within the ink was 9% byweight.

Examples 2 to 5 Comparative Examples 1 to 4

Inks were prepared in the same manner as Example 1 described above,using the formulations shown in Table 2.

The viscosities of the thus obtained inks were all within a range from 9to 12 mPa·s (these viscosity values were measured at 23° C. using acontrolled stress rheometer RS75 manufactured by Haake GmbH (cone angle:1°, diameter: 60 mm), by raising the shear stress from 0 Pa at a rate of0.1 Pa/s, and refer to the viscosity values at 10 Pa). Moreover, all ofthe obtained inks had a pigment particle size that fell within thepreferred range for inkjet inks.

The properties of each ink were evaluated in the manner described below.An HC5500 device (manufactured by Riso Kagaku Corporation) was used asthe inkjet recording apparatus. The HC5500 is a system that uses a 300dpi line-type inkjet head (in which the nozzles are aligned with anapproximately 85 μm spacing therebetween), wherein the paper istransported in a sub-scanning direction perpendicular to the mainscanning direction (the direction along which the nozzles are aligned)while printing is conducted.

<Discharge Stability>

Each ink was loaded into the inkjet recording apparatus, a printingoperation was performed, and the printer was then left to stand in astopped state for 30 minutes. The printing operation was then restarted,and inks for which absolutely no non-discharging nozzles were observedwere evaluated as A, inks which occasionally suffered fromnon-discharging nozzles, but for which these non-discharging nozzlescould be restored to a normal state by performing a typical cleaningoperation (in which an operation of forcing through ink at a pressure of10 kPa for 10 seconds was conducted 3 times) were evaluated as B, andinks for which non-discharging nozzles existed even upon performing atypical cleaning operation were evaluated as C.

<Restoration Performance upon Standing at High Humidity>

Each ink was used to fill the inkjet head, and was then left to standfor 4 weeks in an environment at a temperature of 45° C. and a humidityof 85%. Subsequently, the ink was discharged and the state of the inkdischarge was evaluated. Inks for which the typical cleaning operationdescribed above enabled all of the nozzles to discharge normally wereevaluated as A, inks for which a powerful cleaning operation (in whichan operation of forcing through ink at a pressure of 25 kPa for 10seconds was conducted 3 times) enabled all of the nozzles to dischargenormally were evaluated as B, and inks for which non-discharging nozzlesexisted even upon performing the powerful cleaning operation wereevaluated as C.

<Storage Stability (70° C.)>

Each ink was placed in a sealed container and left to stand for 3 monthsin an atmosphere at 70° C., the variation in the viscosity of the inkwas measured, and the result of that measurement was then evaluated inthe manner described below.

Viscosity Variation:

[(Viscosity after 3 months×100)/(Initial viscosity value)]−100 (%)

Inks for which the viscosity variation was less than 5% were evaluatedas A, inks for which the viscosity variation was at least 5% but lessthan 10% were evaluated as B, and inks for which the viscosity variationwas at least 10% were evaluated as C. This accelerated stability testenables the long-term storage stability of the ink to be evaluated.

<Storage Stability (Room Temperature)>

Each ink was placed in a sealed container and left to stand for 4 weeks,the variation in the viscosity of the ink was measured, and the resultof that measurement was then evaluated in the manner described below.

Viscosity Variation:

[(Viscosity after 4 weeks×100)/(Initial viscosity value)]−100 (%)

Inks for which the viscosity variation was less than 5% were evaluatedas A, inks for which the viscosity variation was at least 5% but lessthan 10% were evaluated as B, and inks for which the viscosity variationwas at least 10% were evaluated as C.

The above results are shown in Table 2.

TABLE 2 Ink Composition and Evaluation Exam- Exam- Exam- Exam- Exam-Comparative Comparative Comparative Comparative ple 1 ple 2 ple 3 ple 4ple 5 Example 1 Example 2 Example 3 Example 4 Pigment dispersant A B C DE F G H I Ink Pigment Pigment 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.045.0 component dispersion dispersant blend solution quantity Carbonblack 27.0 27.0 27.0 27.0 27.0 27.0 27.0 27.0 27.0 Isooctyl palmitate31.5 31.5 31.5 31.5 31.5 31.5 31.5 31.5 31.5 AF7 (naphthene- 31.5 31.531.5 31.5 31.5 31.5 31.5 31.5 31.5 based solvent) AF7 (naphthene-based92.6 90.1 92.6 90.1 92.6 90.1 93.6 92.6 93.6 solvent) AF4(naphthene-based 10.0 12.5 10.0 12.5 10.0 12.5 9.0 10.0 9.0 solvent)Isooctyl palmitate 62.4 62.4 62.4 62.4 62.4 62.4 62.4 62.4 62.4 InkDischarge stability A A A A A C A A B evaluations Restorationperformance upon A A B A A B C A C standing at high humidity Storagestability (70° C.) A A A A B A C C A Storage stability (room A A A A A AC A A temperature)

It is to be noted that, besides those already mentioned above, manymodifications and variations of the above embodiments may be madewithout departing from the novel and advantageous features of thepresent invention. Accordingly, all such modifications and variationsare intended to be included within the scope of the appended claims.

1. A non-aqueous pigment ink comprising a pigment, a pigment dispersantand a non-aqueous solvent, wherein the pigment dispersant is an acrylicpolymer comprising, as constituent monomers, (M1) an alkyl(meth)acrylate having an alkyl group of 12 or more carbon atoms, (M2) a(meth)acrylic acid derivative having an amino group, and (M3)(meth)acrylic acid and/or a (meth)acrylic acid derivative having acarboxyl group, in which a combination of all the constituent monomerscomprises from 8 to 15 mol % of the monomer (M2) and from 0.6 to 8 mol %of the monomer (M3), and the molar ratio (M2)/(M3) between the monomer(M2) and the monomer (M3) is within a range from 1.3 to
 15. 2. A printeditem printed using the non-aqueous pigment ink according to claim
 1. 3.A pigment dispersant for a non-aqueous pigment ink composed of anacrylic polymer comprising, as constituent monomers: (M1) analkyl(meth)acrylate having an alkyl group of 12 or more carbon atoms,(M2) a (meth)acrylic acid derivative having an amino group, and (M3)(meth)acrylic acid and/or a (meth)acrylic acid derivative having acarboxyl group, wherein a combination of all the constituent monomerscomprises from 8 to 15 mol % of the monomer (M2) and from 0.6 to 8 mol %of the monomer (M3), and the molar ratio (M2)/(M3) between the monomer(M2) and the monomer (M3) is within a range from 1.3 to 15.